This invention relates to multi-stage axial flow turbines, and more particularly, to such turbines having an inner and an outer casing.
One of the basic problems facing the turbine designer is introducing high pressure and high temperature elastic fluid such as steam into the main turbine casing before the steam is expanded through the blades of the turbine to do work. The process of expanding the steam produces work and reduces the temperature and pressure of the steam. Containment of the high temperature and pressure steam requires heavy wall containment vessels and casings having very large diameter bolts at horizontal joints as the casing are necessarily made in halves to provide easy access to the internals of the turbine and to facilitate assembly and maintenance. The high and varying temperature steam contained within the casing introduces thermal gradients across the thick walls thereof. The thermal gradients produce differential thermal expansion across the thick walls causing thermal stresses which can produce plastic flow and distortion of the casing. These expansions and resulting distortions must be considered by the designer when setting the clearances between rotatable and stationary portions of the turbine. To reduce the thermal gradients across the thick-walled casings, multi-casing turbines have been developed to break down the pressure and temperature gradients across the individual casings so that each casing, which is free to expand individually, is subjected to lower differential temperature and pressure and thus can be made with thinner walls. Nozzle chambers have generally been disposed within the inner casing requiring a dimensionally large inner casing having heavy walls, additional weight, and higher cost.
U.S. Pat. No. 3,746,463 by Stock et al filed July 26, 1971 and assigned to the assignee of the present invention discloses an inner casing which houses stationary and rotatable blades and is situated within and is supported by the outer casing. A first axial end of the inner casing is open to the outer casing's interior while the second axial end is flexibly sealed to segmented nozzle chambers disposed around the turbine's rotor. Such arrangement has provided very reliable service, but is not amenable to providing complete sealing between the inner casing and the nozzle chambers since the nozzle chambers were segmented. The support arrangements for the inner casing and nozzle chambers provided support for those structures' weight and axial thrust exerted thereon by the expanding steam's reaction forces. Due to such large and costly support arrangements, a reduction in the size thereof and improvement in the sealing between the inner casing and nozzle chambers is desired. Such size reduction would reduce costs and such seal improvement would increase turbine efficiency and provide greater reliability.